Insulation – how to make the right decision
Willow Aliento | 12 March 2015
Insulation is the first line of defence in improving a home’s energy efficiency, according to Dr Richard Aynsley, an expert in sustainable architecture, insulation and interior airflow, but orientation, floorplan, shading and ventilation need to be part of the equation when deciding what types of products to use and where to install them.
Hot climate issues
In hot zones, such as Brisbane, Darwin and Perth, reflective barriers such as aluminium foil insulations are highly effective at preventing radiant heat from entering buildings. These should be installed where the sun hits – generally in hot zone houses this is the roof.
In addition to radiant heat, managing convective and conductive heat is also important. Convective heat is the type that is carried by airflow, and can be reduced through an appropriate combination of building sealing and ventilation.
Conductive heat is the heat transferred by matter, such as a concrete patio that has been absorbing radiant heat and then transfers it to whomever stands on it.
Dr Aynsley says that orientation can assist with reducing heat impacts – for example, locating living areas in the northern part of a home, bedrooms in the east, where heat will not be felt until most people are ready to get up and start the day, and carports and garages on the west to act as a buffer zone for the home.
Shading of walls and glazing is also important to reduce summer heat. In cooler zones, where passive solar heating through glazing to the north is embedded in the Building Code of Australia Section J requirements, he says home owners need to ensure that any estimate of beneficial heat gain is not compromised by glazing being shaded during winter by a high adjacent fence or a close-by neighbouring home. Any shade trees to the north should also be deciduous.
Cool climate insulation issues
One of the critical issues for more temperate climates, Dr Ansley says, is considering condensation risks caused by insulation and building sealing.
Dr Aynsley was one of the contributors to the Master Builders Association and Australian Institute of Architects 2011 Building Codes Handbook Condensation in Buildings.
The handbook says a lowering of the temperature within wall and roof spaces can increase the risk of condensation, which has a range of risks including the growth of toxic moulds and damage to building materials such as rotting of studs, corrosion of metal fastenings or “leaky building syndrome”.
“Interstitial condensation can be far more damaging to the building than surface condensation. Interstitial condensation can go unnoticed and if the building fabric has not been designed to allow moisture to dry from within it can become trapped and compromise the durability of the building and the health of the occupants,” the handbook says.
Higher levels of insulation are actually creating cooler external surfaces in temperate areas, leading to condensation occurring within the wall cavity in some cases. In warm climates, where insulation and airconditioning create cooler internal surfaces relative to external ones, it can occur inside the dwelling.
Dr Aynsley says airconditioning in the tropics can be a major contributor to condensation occurring and can promote the growth of toxic moulds that pose genuine risks to human health.
Another issue identified in the handbook is that interstitial condensation can build up within fibre-based insulation, and that at even one per cent moisture content their effectiveness is reduced as water is a better conductor of heat than air pockets.
“The closed cell structure of some foam insulation materials such as extruded polystyrene, phenolic, polyisocyanurate (PIR) and polyurethane (PUR) insulation are less susceptible to moisture and water vapour ingress and so are less prone to loss of insulative performance. Open cell materials such as mineral wool and expanded polystyrene are more at risk of loss of thermal resistance,” the handbook says.
And the warmer the interior is, the more moisture the air can hold and the more likely “dew point”, the temperature at which water will condense out of air, will be reached.
Melbourne-based architect and former project manager built environment for Sustainability Victoria Ande Bunbury told The Fifth Estate that dew point is something she assesses for every room in a building she designs, as the risk of it occurring is greater where the exterior temperature is very low and the interior kept warm.
To mitigate the risk of condensation, where her assessment shows dew point is likely to be reached within the homes normal climatic and internal heating and cooling parameters, she adjusts the design of elements such as battens or adds extra sarking [an aluminium foil backed building paper for insulation and weather proofing] to the walls.
In cooler climates like Victoria’s where energy use for heating needs management, she says bulk insulation is an effective choice.
“Insulation is about creating stationary air pockets,” she says.
Bunbury, who is focused on sustainable design, says she prefers to specify polyester batts for bulk insulation in walls and roofs as it is possible to obtain products that are manufactured from recycled plastic bottles, and the insulation is fully recyclable at end of life. They are also low-irritant and do not off-gas any volatile organic compounds or other toxins.
There have also been developments in polystyrene composite board products, she says. This is panellised clad polystyrene that can be used as a structural element. In addition to reducing materials through being a combination of finishes, insulation and structural material, they also save considerable time during the building process.
Bunbury says that insulation is not something to be considered on its own, and that thermal mass and orientation are also important, as is the use of stack ventilation principles.
Other elements that can contribute to thermal performance include double-skinned roofs, and Trombe walls, such as a dark-painted wall with window that absorbs heat during the day in winter, and radiates it back out at night.
Landscaping around the house can be used to pre-cool air before it enters the house, through putting water features or vegetation on the prevailing wind side of a dwelling.
The dual climate
Aluminium reflective foil firm Wren Industries’ director Tim Renouf told The Fifth Estate that in a dual climate, such as Canberra’s, where summers are high in radiant heat and winter temperatures plummet, a combination of reflective foil and bulk insulation in the roof is best.
The foil barrier not only prevents the majority of radiant heat during summer entering the home, it will also protect the bulk insulation from being damaged by the kinds of extreme high temperatures that commonly occur within Australian roof spaces.
Aluminium foil insulation comprises a layer of pure aluminium about six microns thick laminated to either plastic or paper. It has been in use in Australia since the 1960s. It is excellent at blocking radiant heat from moving downwards into a home, but will not stop heat escaping from the interior via the roof in winter.
Bulk insulation works on the principle of trapping numerous tiny pockets of stationary air, just like a feather doona. Types of bulk insulation in the typical “batt’ form include fibreglass, rockwool, polyester, natural wool, and cellulose-based insulations. Batts work like a sponge to either absorb heat or cold.
Spray-in insulations – a number of firms are now providing a form of cellulose-based bulk insulation which is a loose-fill type, applied via a spray-in or pump-in process. These work in a similar manner to batts, however, there needs to be care taken to ensure appropriate physical barriers are put in place within roof cavities or walls to stop insulation from causing the overheating of light fixtures, exhaust fans or electrical cabling. The barriers need to be put in place around where each specific fixture is located. While the majority of these insulations seem to include fire-retardant chemicals, such as borax or boric acid, this does not prevent overheating setting a roof truss on fire.
Phase change materials – these use a material such as a wax to absorb heat. Basically, encapsulated particles such as micronised wax will absorb heat and melt when the temperature rises, and then release the heat and return to solid state when they cool. Charles Sturt University at Albury-Wodonga installed a form of this type of insulation in the floors of a new administration building in 2011.
Bunbury says there is another version on the market which uses match-box sized blocks of plant-derived material within a paper-based sheeting that is suitable for walls. She says they emulate thermal mass, making them a good insulation choice for lightweight construction. The limitation with them is that once they have melted, they can’t absorb any more heat.
David Goodfield, research associate and operations manager at Murdoch University’s Environmental Technology Centre, researched the effectiveness of phase change materials for a postgraduate thesis in 2008. He designed and constructed a prototype lightweight home and incorporated a plasterboard with incorporated phase change micronised wax.
He told The Fifth Estate that his project showed the importance of ventilation and night purging in removing the heat emitted from the materials during the cooler part of the night in summer.
Ventilation systems are also helpful in using the material as a means of distributing heat, with it first warmed by being installed in proximity to gas heating, and ventilation used to distribute the retained warmth once heating is turned off.
The big things you should consider before any insulation retrofit
As the Royal Commission into the Home Insulation Program found, existing homes in Australia are likely to have risks in the roof space. Before any insulation retrofit is carried out, an inspection of the roof space by a licensed electrician is essential to ensure the safety of installers and to reduce the risk of post-installation fires or electrocution.
New homes also should receive an independent inspection of in-roof electrical wiring to ensure the appropriate standards were met.
More important points from the RC:
- Insulation installation should be undertaken in a manner that allows anyone entering the roof space in future to still see where the joists are, as most ceiling materials are not designed for walking on and the risk of falling through is very real unless someone walks on the joists.
- Installation of insulation involves two risky work situations – working at heights, and working in enclosed spaces. Roof cavities can also become extremely hot, and are often devoid of any form of lighting or ventilation. Care needs to be taken to manage heat stress risks for installers, as well as managing the height and enclosed space risks.